Here we seek to understand how structural flexibility and variation in parvoviral capsids control their ability to bind receptors leading to cell infection and also to variation in host range, and also how capsid structures control antibody binding and neutralization. Those areas of study are significant because they are features of all animal and human viruses. While parvovirus capsids appear structurally simple, they are clearly sophisticated biomolecular machines that carry out many functions using variants of a single capsid protein, and the features controlling many functions have now been mapped to specific mutations and capsid structures, presenting an opportunity to gain a complete understanding of how virus-host interactions occur in fine detail. Parvoviruses include the B19 virus, human bocavirus, and Parv4, all of which cause disease in humans. Here we use feline and canine parvoviruses as models to build on our previous studies showing that cell infection and animal host ranges are controlled by specific interactions of the capsids with the transferrin receptors type-1 (TfR) of different hosts. There are also distinct outcomes for viral infection of antibody binding, depending on the binding site and angle of attachment. The three overlapping areas to be investigated in this project are: 1. Define the functional effects of variant and flexible structures of the parvovirus capsids. There is both flexibility and structural variation in the viral capsids which we will further characterize in detail. Some of that variation is asymmetric, only occurring in a small proportion of the capsid subunits, while other variation occurs in most viral sites. Many of those sites are known to affect viral functions. 2. Characterize how different interactions between parvovirus capsids and TfR or other receptors control infection. Capsid binding to different TfRs is controlled by the protein structure, and flexibility, and potentially by protease cleavages. To define the functional TfR binding to the capsids we will use a variety of approaches, including cryoEM analysis and analysis of capsid mutants affecting binding. To define the TfR interaction we will use receptors containing amino acid changes or added glycans within the interaction sites. 3. Use antibody binding to capsids to define their structures, and also to explain the mechanisms of antibody neutralization. We have many antibodies that bind the capsid structures. Some neutralize as Fabs, and others do not. We will use those antibodies as probes for structural variation in the capsids. The antibodies and their domains will be used in competition assays to further define the binding of different TfRs to the capsids, and to detect cleaved subunits or other capsid modifications. Antibodies with altered binding affinities, different attachment sites, or different angles of attachment would be tested for effects on the virus functions involved in cell infection.